Wavelength division multiplexing source using multifunctional filters

ABSTRACT

This invention provides a system that combines a wavelength multiplexer with an FM discriminator for chirp reduction and wavelength locker in a filter to produce a wavelength division multiplexed signal with reduced chirp. A partially frequency modulation laser signal is converted into a substantially amplitude modulation laser signal. This conversion increases the extinction ratio of the input signal and further reduces the chirp. A wavelength division multiplexing (WDM) method is used for transmitting high capacity information through fiber optics systems where digital information is carried on separate wavelengths through the same fiber. Separate transmitters normally generate their respective signals that are transmitted at different wavelengths. These signals are then combined using a wavelength multiplexer to transmit the high capacity information through the fiber optic system. Various technologies can be used to multiplex the signals such as, for example, thin film filters, or arrayed waveguide gratings. In a WDM system, a wavelength locker may also be used that fixes the center wavelength of a transmitter to a reference. Wavelength lockers may include etalons or fiber gratings, either of which provides a reference wavelength. A control circuit typically compares the wavelength of the transmitter to the reference. An error signal adjusts the transmitter format wavelength by varying temperature or by other means to keep it locked to the reference wavelength.

RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Applicationserial No. 60/395,073, entitled “wavelength division multiplexing sourceusing multifunctional filters,” which was filed Jul. 9, 2002, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention addresses a multi-wavelength fiber optictransmitter related to directly modulated laser sources andmultifunctional filters.

[0004] 2. General Background and State of the Art

[0005] Fiber optic communication systems use a variety of transmittersto convert electrical digital bits of information into optical signalsthat are carried by an optical fiber to a receiver. In a directlymodulated transmitter the output intensity of a laser is modulated bydirectly changing the injection current driving the laser. In anexternally modulated transmitter, the intensity of a continous wavelaser is modulated via the use of a modulator, which changes theintensity of the laser light. Directly modulated semiconductor lasersare typically compact, integrable, and have large responses tomodulation. They are comparatively inexpensive than externally modulatedtransmitters, which require an intensity modulator following the lasersource. However, directly modulated lasers may suffer from a drawback;namely, their outputs may be highly chirped. As a result, directlymodulated lasers are normally used for short reach applications becausethe inherent chirp of the laser causes the transmitted pulses to bedistorted after propagation in dispersive fiber. For longer reachapplications, external modulation is used. However, external modulationrequires a costly modulator that consumes power, introduces loss, andtakes up board space.

INVENTION SUMMARY

[0006] This invention provides a system that combines a wavelengthmultiplexer with a frequency modulated (FM) discriminator for chirpreduction and wavelength locker in a filter to produce a wavelengthdivision multiplexed signal with reduced chirp. A FM modulated laser andan optical discriminator as described in U.S. Pat. No. 6,104,851 may beused with this invention, which is incorporated by reference into thisapplication. In this technique, the laser is initially biased to acurrent level high above threshold. A partial amplitude modulation (AM)of the bias current is affected such that the average power outputremains high. The partial amplitude modulation also leads to a partialbut significant modulation in the frequency of the laser output,synchronous with the power amplitude changes. This partially frequencymodulated output may then be applied to a filter, such as a thin filmfilter or a fiber Bragg grating, or any type of filter known to one inthe art, which is tuned to allow light only at certain frequencies topass through. This way, a partially frequency modulated signal isconverted into a substantially amplitude modulated signal. Simply,frequency modulation is converted into amplitude modulation. Thisconversion increases the extinction ratio of the input signal andfurther reduces the chirp.

[0007] A wavelength division multiplexing (WDM) method is used fortransmitting high capacity information through fiber optics systemswhere digital information is carried on separate wavelengths through thesame fiber. Separate transmitters normally generate their respectivesignals that are transmitted at different wavelengths. These signals arethen combined using a wavelength multiplexer to transmit the highcapacity information through the fiber optic system. Varioustechnologies can be used to multiplex the signals such as, for example,thin film filters, or arrayed waveguide gratings.

[0008] In a WDM system, a wavelength locker may also be used that fixesthe center wavelength of a transmitter to a reference. Wavelengthlockers may include etalons or fiber gratings, either of which providesa reference wavelength. A control circuit typically compares thewavelength of the transmitter to the reference. An error signal adjuststhe transmitter wavelength by varying temperature or by other means tokeep it locked to the reference wavelength.

BRIEF DESCRIPTION OF THE FIGURES

[0009]FIG. 1 illustrates a WDM source including distributed feed-back(DFB) laser sources multiplexed by filters that operate also as opticaldiscriminators and wavelength lockers.

[0010]FIG. 2 illustrates an optical output of a DFB laser before andafter the filter for non-return-to-zero (NRZ) data modulation.

[0011]FIG. 3 illustrates an optical spectrum of the laser and filter inoperating condition of the device, in reflection.

[0012]FIG. 4 illustrates an optical spectrum of the laser and filter inoperating condition of the device, in transmission.

[0013]FIG. 5 illustrates a wavelength locking circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014]FIG. 1 illustrates a system 100 capable of producing a wavelengthdivision multiplexed (WDM) source for long reach applications wheremultiple appropriate filters may be used for multiplexing, opticalfrequency discrimination, and wavelength locking. The system 100 mayinclude a plurality of current modulators 101, 102 and 103, coupled to aplurality of laser sources 201, 202, and 203, respectively. Each of thecurrent modulators may directly current-modulate the digital signalsprovided from the corresponding laser sources. The system 100 may alsoinclude optical isolators 301, 302, and 303 on the output side of therespective laser sources 201, 202, and 203. The optical isolators may beincorporated into the system 100 to prevent optical feedback into thelasers, which can degrade their performance. The system 100 includesfilters 401, 402, and 403, which are positioned in such a way thatwavelength channel from one laser source is reflected in the samedirection as the transmitted light from another laser source.

[0015] The laser sources may be laser diode chips, each capable ofproducing a different wavelength signal than the other. The plurality oflaser diode chips such as 201, 202, and 203, each having a differentwavelength may be multiplexed using filters 401, 402, and 403,respectively. The filters may be substantially matched in wavelength tothe lasing frequency of the single mode laser diode. The laser diodesmay be distributed feedback lasers with stable single mode operation.The filters may be designed so that they transmit a narrow band ofwavelengths near a central wavelength, and reflect most or all otherwavelengths. The filters may be positioned relative the directions ofdifferent lasers to transmit the wavelength of the laser to bemultiplexed. For example, the position or angle of each filter may beadjusted in such a way to reflect most or all other wavelength channelsfrom the other lasers into the same direction as the transmitted lightfrom the first laser. In this way the optical signals from a number ofsources with different wavelengths may be directed into a common port;i.e. multiplexed.

[0016] A multiplicity of such laser outputs can be directed to the sameoutput port using a number of similarly placed filters as illustrated inFIG. 1. In this example, a filter 405 may be provided to convert a laserhaving a wavelength λ₅ with a partially frequency modulated signal to asubstantially amplitude modulated signal with wavelength λ₅. In thisregard, U.S. patent Ser. No. 10/289,944 entitled “Power Source for aDispersion Compensation Fiber Optic,” filed Nov. 6, 2002, whichdiscloses converting frequency modulated signal to amplitude modulatedsignal, is incorporated by reference. The filter 404 may be positionedto reflect the laser having λ₅ wavelength in substantially samedirection as the laser having λ₄ wavelength, resulting in a firstmultiplexed signal with wavelengths λ₄ and λ₅. The filter 403 ispositioned to reflect the first multiplexed signal in substantially samedirection as the laser having λ₃ wavelength, resulting in a secondmultiplexed signal with wavelengths λ₃, λ₄, and λ₅. The filter 402 ispositioned to reflect the second multiplexed signal in substantiallysame direction as the laser having λ₂ wavelength, resulting in a thirdmultiplexed signal with wavelengths λ₂, λ₃, λ₄, and λ₅. The filter 401is positioned to reflect the third multiplexed signal in substantiallysame direction as the laser having λ₁ wavelength, resulting in a fourthmultiplexed signal with wavelengths λ₁, λ₂, λ₃, λ₄, and λ₅. Accordingly,the optical signals from a number of sources with different wavelengthsmay be directed into a common port or multiplexed.

[0017] These filters may be produced by the deposition of multiplelayers of a dielectric material on a transparent substrate. Softwaretools may allow one skilled in the art to design a filter with a desiredtransmission profile, by choosing the various layers of the dielectric.

[0018]FIG. 2 illustrates the modulated signal of any one of the diodelasers which may be directly current-modulated by a digital signal usingcurrent modulators 101, 102, or 103, for each of the respective laserdiodes chips, while biased high above their respective thresholdcurrents. This biasing condition may produce optical signals 501 withlow extinction ratio, but with low residual chirp. The extinction ratiobefore the filter may be about 2-4 dB. In this case, the output may havea large frequency modulation in addition to amplitude modulation becauseof the inherent linewidth enhancement effect in semiconductor lasers.FIG. 2 illustrates the frequency excursion 502 of the output as afunction of time for a non-return-to-zero NRZ signal under the conditionthat transient chirp may be low compared to the adiabatic chirpcomponent. Transient chirp may be associated with the edges of thepulses, while adiabatic chirp may be the frequency excursion for thequasi-steady state 1 and 0 levels. The filter may convert this frequencymodulation to amplitude modulation, producing an optical signal 503having an enhanced extinction ratio higher than 10 dB. The resultingsignal may also have low chirp. This may be done by keeping the laserhigh above threshold to minimize large residual chirp and transientringing in the case of NRZ data modulation. The modulated output of thelaser may have a return-to-zero (RZ) format in which the signal returnsto zero between consecutive Is. In an NRZ signal the optical signalremains high (does not return to zero) between consecutive Is. Thesignal modulating the signal may also be a sinusoidal RF signal. In thiscase the discriminator may convert the sinusoidal input to opticalpulses.

[0019]FIGS. 3 and 4 show the transmission 601 and reflection 603,respectively, of a filter that may be used with this invention. Theremay be a band in wavelength over which the filter transmits most of thelight in that wavelength range, while most or all of the wavelengthsoutside that band are reflected. The sum of transmission and reflectionmay be nearly 100%. For this purpose, the filter's edge 604 may be usedwhere reflection and transmission vary as a function of frequency orwavelength. The laser spectrum 602 is also shown at the output relativeto the filter shape. The signal spectrum may be substantially near thefilter edge 604 when the transmission vs. frequency slope is high,typically about 1 dB/GHz. The edge of the filter may act as an opticaldiscriminator and may be used to convert frequency modulation of thelaser to amplitude modulation to produce low-chirp optical output withhigh extinction, as described below and in U.S. Pat. No. 6,104,851 andreferences therein. In the case of NRZ modulation as illustrated in FIG.2, the laser spectrum may be tuned to be on the long wavelength edge ofthe filter spectrum such as to transmit the blue-shifted components ofthe laser output and reflect the red-shifted components.

[0020] In the case of FIG. 2, the blue-shifted components 504 may be 1bit while the red-shifted components 505 may be 0 bits, which arerelatively red-shifted compared to the average wavelength of the laseroutput. In the case of RZ modulation, the blue-shifted components, whichare coincident with the rising edges of the optical pulses may betransmitted, while rest is reflected.

[0021]FIG. 5 illustrates a system 700 for using filters tosimultaneously lock the wavelengths of the multiplicity of laser diodes.The lasers 201 and 202, and the filters 401 and 402, may be mounted onseparate thermo-electric coolers (TECs) 701, 702, 801, and 802,respectively. A first set of photodiodes 711 and 712 may monitor theoptical power at the back facet of the lasers 201 and 202, respectively.A second set of photodiodes 811 and 812 may monitor power reflected fromthe filters 401 and 402, respectively. The system 700 also includes awavelength locking circuit 901 having a number of independent circuitsfor each laserdiode/filter pair. Each circuit, such as 901, may includea comparator 903 that compares the ratio of the signals (taken usingdivider circuit 902) from the PD_(filter) 811 to the PD_(laser) 711,r=P_(reflected)/P_(laser), to a fixed, set value or a reference value904. The error signal produced in this way may then control the laserTEC 701 to adjust the laser temperature and therefore shift the laserwavelength in order to keep r substantially constant.

[0022] In FIGS. 3 and 4, if the laser wavelength drifts to longerwavelengths due to aging, for example, P_(reflected) increases relativeto P_(laser), increasing the value of r relative to the reference value.The circuit may then cause the laser to be cooled slightly, shifting itswavelength to shorter wavelengths. This in turn decreases P_(reflected)and decreases the ratio r back towards the reference value. The laserwavelength may be substantially locked to the transmission edge of thefilter. To avoid wavelength drift, the temperature of each filter may befixed by separate thermoelectric coolers, 811 and 812 and correspondingtemperature sensors 821 and 822. Note that for each additionaldiode/filter pair, an electric circuit, such as 901, may be used tosubstantially lock the transmission edge of the filter.

[0023] While various embodiments of the invention have been described,it will be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of thisinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents.

What is claimed is:
 1. A fiber optic communication system, comprising: afirst optical discriminator positioned to convert a first partiallyfrequency modulated signal of wavelength λ₁ into a first substantiallyamplitude modulated signal and to reflect a multiplicity of multiplexedsignals with wavelengths λ₂, . . . , λ_(n), which are different from λ₁,so that the first substantially amplitude modulated signal of wavelengthλ₁ and the multiplicity of multiplexed wavelengths λ₁, λ₂, . . . λ_(n)are made to propagate in substantially the same direction to form awavelength multiplexed signal with wavelengths λ₁, λ₂, . . . , λ_(n). 2.The system according to claim 1, in which the multiplexed signals withwavelengths λ₁, . . . λ_(n), are generated by another multiplicity offiber optic system.
 3. The system according to claim 1, where theoptical discriminator is adapted to reflect a portion of the partiallyfrequency modulated signal to produce a reflected signal that is used towavelength lock the partially frequency modulated signal.
 4. The systemaccording to claim 3, further including a wavelength locking circuitadapted to wavelength lock the partially frequency modulated signal bycomparing a first optical power of the partially frequency modulatedsignal to a second optical power of the reflected signal and thenadjusting the partially frequency modulated signal to keep the ratio ofthe partially frequency modulated signal to the reflected signalsubstantially constant.
 5. The system according to claim 1, where thefirst optical discriminator partially compensates for dispersion in atransmission cable.
 6. The system according to claim 1, furtherincluding a laser source to provide the first partially frequencymodulated signal of wavelength λ₁, and an optical isolator between thelaser source and the first optical discriminator.
 7. The systemaccording to claim 1, the optical discriminators are each coupledmulticavity filters.
 8. The system according to claim 1, the opticaldiscriminators are formed from a stack of thin materials havingdifferent dielectric constants.
 9. The system according to claim 1,where the modulating signal is non-return to zero.
 10. The systemaccording to claim 1, where the modulating signal is return to zero. 11.The system according to claim 1, where the modulating signal issinusoidal RF signal.
 12. A fiber optic communication system,comprising: a first optical discriminator adapted to convert a firstpartially frequency modulated signal into a first substantiallyamplitude modulated signal; a second optical discriminator adapted toconvert a second partially frequency modulated signal into a secondsubstantially amplitude modulated signal and to reflect the firstsubstantially amplitude modulated signal so that the first substantiallyamplitude modulated signal and the second substantially amplitudemodulated signal are substantially in the same direction to form a firstwavelength multiplexed signal.
 13. The system according to claim 12,further including a first wavelength locking circuit adapted towavelength lock the first partially frequency modulated signal bycomparing a first optical power against a second optical power of thereflected signal of the first partially frequency modulated signal andthen adjusting the first partially frequency modulated signal to keepthe ratio of the first partially frequency modulated signal to thereflected signal substantially constant.
 14. The system according toclaim 13, further including: A third optical discriminator adapted toconvert a third partially frequency modulated signal into a thirdsubstantially amplitude modulated signal and to reflect the firstwavelength multiplexed signal so that the third substantially amplitudemodulated signal and the first wavelength multiplexed signal aresubstantially in the same direction to form a second wavelengthmultiplexed signal.
 15. The system according to claim 14, where thethird optical discriminator is adapted to reflect a portion of the thirdpartially frequency modulated signal to produce a third reflected signalwhich is used to wavelength lock the third partially frequency modulatedsignal.
 16. The system according to claim 12, further including a lasersource to provide the first partially frequency modulated signal, and anoptical isolator between the laser source and the first opticaldiscriminator.
 17. The system according to claim 16, where the lasersource is a semiconductor laser diode.
 18. A fiber optic system capableof multiplexing, the system comprising: a first laser source capable oftransmitting a first partially frequency modulated (FM) laser signal; afirst optical discriminator adapted to convert a first partially FMlaser signal into a first substantially amplitude modulated (AM) lasersignal; a second laser source capable of transmitting a second partiallyFM laser signal, where the wavelength of the first partially FM lasersignal is different from the wavelength of the second partially FM lasersignal; and a second optical discriminator positioned relative to thefirst and second laser sources such that the optical discriminatorconverts the second partially FM laser signal to a second substantiallyAM laser signal and reflect the first substantially AM laser signal sothat the first and second substantially AM laser signals propagate insubstantially the same direction to form a first wavelength multiplexedlaser signal.
 19. The system according to claim 18, further including awavelength locking circuit adapted to wavelength lock the firstpartially FM laser signal by comparing a first optical power of thepartially FM laser signal to a second optical power of the reflectedsignal and then adjusting the partially FM laser signal to keep theratio of the partially FM laser signal to the reflected signalsubstantially constant.
 20. The system according to claim 19, where thefirst laser source is coupled to a laser cooler, if the second opticalpower of the reflected signal increases relative to the first opticalpower, then cooling the laser cooler to shift the wavelength of thefirst substantially AM laser signal to be shorter.
 21. The systemaccording to claim 19, where the first optical discriminator is coupledto a discriminator cooler to fix the temperature of the discriminator tominimize wavelength drift.
 22. A fiber optic communication system,comprising: means for providing a first partially frequency modulated(FM) laser signal that is different from the wavelength of a secondpartially FM laser signal; means for converting the first and secondpartially FM laser signal to respective first and second amplitudemodulated (AM) laser signals; means for multiplexing the first andsecond partially AM laser signals to propagate in substantially the samedirection.
 23. A method for multiplexing at least two signals, themethod comprising: converting a first partially frequency modulated (FM)laser signal to a first substantially amplitude modulated (AM) lasersignal; and reflecting the first substantially AM laser signal insubstantially the same direction as multiplexed laser signals withdifferent wavelengths as the wavelength of the first substantially AMlaser signal.
 24. The method according to claim 23, further including:cooling the first substantially AM laser signal to shorten itswavelength to decrease the power of the laser from the step ofreflecting.